The present invention relates to the field of electronic devices and their manufacture. More specifically, in one embodiment the invention provides a method and apparatus for limiting current flow for the purpose of, for example, protection, control, signal generation, and the like.
Often it is desirable to provide a means and method for limiting current from a variable voltage source in an electronic circuit. Such current limiters (otherwise referred to as current regulators, current sinks, constant current sources, and the like) could be used in a wide variety of applications which include, but are by no means limited to, protection, control, signal generation.
Ideally, a current limiter provides an output current which is largely independent of an input voltage. In reality, most such devices do not begin to operate effectively until the voltage between their terminals reaches some minimum voltage, generally referred to as V.sub.t. Their performance begins to deteriorate above some breakdown voltage, V.sub.bd.
FIG. 1 illustrates a typical current limiting device 2. The current I.sub.s is to be regulated. The current limiting device includes an enhancement mode HVFET 4 which is driven in a closed loop mode via a shunt resistor 6 and an amplifier 8. The current I.sub.s will, within limits, be set by the resistance of the shunt resistor 6 and will be largely independent of the voltage applied at the point "D". I.sub.s will equal approximately V.sub.t /R.sub.s , where V.sub.t is roughly the threshold voltage of the FET 4. Such current limiting devices have the disadvantage of, for example, being complex. In the device shown in FIG. 1, for example, it is necessary to provide a power supply for V.sub.cc, an op-amp 8, a transistor 4, a resistor 6, and the like.
FIG. 2 illustrates another type of current source in which a depletion mode FET 10 is utilized. In the device shown in FIG. 2, the current is "on" when the gate of the FET is shorted to the source. I.sub.D is set by processing conditions in the fabrication of the device and, in particular, the design of the FET and the resistors. Accordingly, users of such devices frequently purchase a transistor 10 and select a discrete resistor 6 to provide the appropriate current I.sub.D. Often, for different transistors, even those fabricated in the same production facility, it is found that different values of R.sub.s must be provided to obtain a desired value of I.sub.d.
An improved method and apparatus for providing a constant current is desired.